Cross-linked carboxyl-group containing polymers have been conventionally used as thickeners for various aqueous solutions. As these cross-linked carboxyl-group containing polymers, there have been known, for instance, a copolymer of an α,β-unsaturated carboxylic acid such as acrylic acid with a polyallyl ether (U.S. Pat. No. 2,923,692); a copolymer of an α,β-unsaturated carboxylic acid with hexaallyltrimethylene trisulfone (U.S. Pat. No. 2,958,679); a copolymer of an α,β-unsaturated carboxylic acid with triallyl phosphate (U.S. Pat. No. 3,426,004); a copolymer of an α,β-unsaturated carboxylic acid with glycidyl methacrylate or the like (Japanese Patent Laid-Open No. Sho 58-84819); and the like.
These cross-linked carboxyl group-containing polymers have been used for applications such as thickeners and suspension stabilizers for emulsions and suspensions by dissolving the polymer in water and neutralizing the solution with an alkali to give a neutralized viscous solution.
In order to use the above-mentioned cross-linked carboxyl group containing polymer for these applications, it is necessary to prepare a homogeneous aqueous solution of the polymer. However, when the cross-linked carboxyl-containing polymer is dissolved in water, undissolved powder lumps tend to be generated, and once the undissolved powder lumps are generated, a gel-like layer is formed on its surface. Therefore, there are some defects such that the rate of penetration of water into the internal of the polymer is delayed, thereby making it difficult to obtain a homogeneous solution. Therefore, when the above-mentioned cross-linked carboxyl group-containing polymer is used, there is necessitated gradual addition of the cross-linked carboxyl-group containing polymer to water under high speed agitation, which is an operation low in production efficiency, in order to prevent the generation of undissolved powder lumps, and in some cases, there is a defect such that a specialized dissolving device for prevention of the generation of undissolved powder lumps is necessitated.
In addition, the higher the viscosity of the above-mentioned neutralized viscous solution is, the wider the applications for thickeners become, and the amount of the viscous solution can be reduced. Therefore, in recent years, it has been earnestly desired to develop a polymer for thickeners giving high viscosities.